The present invention relates to apparatus for making plastic composites, and more particularly to such apparatus for making a thermoplastic composite.
Conventional devices for making composites from thermoplastic resins are expensive and complicated compared to those for making composites from thermoset resins. The key reason for such distinction is the relatively high viscosities associated with thermoplastic resins compared to thermoset resins. The higher the resin viscosity, the more difficult it is to achieve the required degree of wetting of filler to produce, ultimately, a uniform, cohesive end product.
There remains a need to provide improved apparatus for making a thermoplastic composite because thermoplastic resins are substantially less expensive than thermoset resins. In addition, thermoplastic resins are sometimes preferred because they impart certain desired properties to the composite, such as desired compressive strength, wear resistance, etc.
Presently, there are at least two known systems of mixing thermoplastic polymers with powder-like fillers. In one system, solid thermoplastic pellets and filler powder are pre-mixed and then passed through a heated extruder where the pellets are melted, the pellets and powder are mixed by a mechanical device, and the molten composite is ejected. In another system, heat and extremely high pressure are used to force the mixture of thermoplastic pellets and filler powder through a die and into a mold.
To mix thermoplastic polymers with non-powdered fillers, there are at least three conventional systems. In one system, filler particles are individually dipped in a hot viscous bath of thermoplastic resin and then, after cooling, the dip-coated filler particles are woven into a fabric-like form. Next, such fabric-like material is positioned in a mold with additional thermoplastic resin. Heat is then applied causing the additional thermoplastic resin to melt into and around areas of the fabric-like material to fill in dry spots where the dip-coating step may not have sufficiently wetted the material.
An alternative system involves using extremely high-pressure injection of thermoplastic material into a mold to coat certain types of fillers. A limited number of fillers are usable in such systems because of such extreme environment.
It is also known to make relatively thin sections of composite material by layering thermoplastic pellets and filler material in a mold followed by heating the mold.
Accordingly, it is a principal object of the present invention to provide apparatus for making a thermoplastic composite that overcomes the drawbacks, such as those just mentioned, associated with conventional systems.
Another object of the present invention is to provide such apparatus for making a thermoplastic composite that is a solid, homogenous, non-foamed material that can be easily and relatively inexpensively manufactured.
A still further object is to provide the described apparatus that will provide optimal heating efficiency with respect to heating the thermoplastic/filler mixture, thus to convert the mixture into a flowable state that promotes proper bonding and ultimately, improved forming of the thermoplastic composite.